Method of producing an electrical capacitor



METHOD OF PRODUCING AN ELECTRICAL CAPACITOR Original Filed May 5, 19592e, 1; 24 I 22 14 i- I I l8 1 3 INVENTORS lyneamezsr fl Lnyron Reamer 6.Ben/054s Arron/v5) United States Patent 3,292,234 METHOD OF PRODUCING ANELECTRICAL CAPACITOR Margaret M. Layton, Corning, Robert B. Randels,Painted Post, and George F. Stockdale, Corning, N.Y., asslgnors toCorning Glass Works, Corning, N.Y., a corporanon of New York Originalapplication May 5, 1959, Ser. No. 811,186, now Patent No. 3,169,216,dated Feb. 9, 1965. Divided and this application May 15, 1964, Ser. No.374,222 2 Claims. (C1. 29-25.42)

This application is a division of Serial No. 811,186, filed May 5, 1959,now Pat. No. 3,169,216.

This invention relates generally to electrical capacitors of the typedescribed in US. Patent No. 2,696,577 issued to G. P. Smith. It isarticularly concerned with improvements in capacitor encapsulation andelectrical lead design to provide a hermetically encapsulated capacitor.

In the glass capacitor of the Smith patent, thin strips of glass areemployed as dielectric material and are interleaved with metal foilelectrodes to form a stack. The stack is encased between shaped glasscovers, and the entire assembly consolidated by application of heat andpressure to produce a glass encapsulated glass capacitor.

Such capacitors provide superior electrical properties and a compactintegral construction well adapted to miniaturized electrical systems.Experience has demonstrated, however, that utilization under certainenvironmental conditions requires a combination of properties which hasheretofore appeared incompatible. Among these propertie are relativelyflexible electrical leads, a high Q value in addition to otherelectrical characteristics, and means for excluding moisture from theencapsulated capacitor.

Flexible electrical leads facilitate assembly of capacitors intoelectrical systems and minimize transmission of stress to the fragilecapacitor element and encapsulation. However, the conventional flexiblewire materials, brass and copper, have thermal coefficients of expansionmuch higher than ordinary glasses. Adhesion of the cover glass to such awire lead during the consolidation step causes stresses to develop oncooling. These stresses result in glass fracture, and it has, therefore,been considered necessary to avoid such adhesion in producing glassencased capacitors in accordance with the Smith patent.

Under conditions of high humidity, saltwater spray and the like,moisture enters the capacitor assembly along the lead Wire. This hasbeen found to promote an electrochemical reaction at the terminaljunction which results in failure of the capacitor. Organic sealantshave failed to provide a complete barrier to such moisture entry.

Among the various electrical characteristics of capacitors, prescribedin terms of minimal limits by specifications such as militaryspecification MIL-C-ll272A, is the capacitor Q value. As is well known,this value is a dimensionless figure of merit for capacitors that variesinversely with the resistance imparted by various capacitor assemblycomponents. It is numerically equal to the ratio of reactance toresistance and may also be expressed as the reciprocal of thedissipation factor. V

In glass capacitors, optimum Q values are obtained by producing theglass dielectric film and the encapsulating covers from a glass of thetype disclosed in US. Patent No. 2,527,693 issued to W. H. Armistead.These glasses have coefiicients of expansion on the order of 100-1O5 l0'cm./cm./ C. It is known that several ferrous alloys possess coefficientsof thermal expansion sufficiently close to that of such a glass topermit a fusion seal between the glass and wires produced from thesealloys. In attempting to use lead wires produced from such alloys,however, it was found that the ultimate capacitor Q values wereprohibitively low except as the wires were of such large diameter as tobe essentially inflexible.

It is a primary purpose of this invention to provide an improvedcapacitor construction and method of production whereby these problemsare avoided and the described requirements are fulfilled. A furtherpurpose is to provide a hermetically encapsulated capacitor. Anotherpurpose is to provide an improved electrical lead for encapsulatedcapacitors. A more specific purpose is to provide a glass encapsulatedcapacitor having a strong, crack-resistant construction and a fusionseal between the electrical leads of the capacitor and the glassencapsulation. A further purpose is to provide an improved method ofproducing encapsulated capacitors.

The present invention provides a hermetically encapsulated capacitorcomprising a capacitor element composed of electrodes separated bydielectric material, electrically conducting leads attached to theelectrodes and extending outwardly through a glass casing thatencapsulates the element, each such lead being composed of a relativelyinflexible portion, such portion being attached to an electrode andfusion-sealed into the glass casing and having an effective thermalcoeflicient of expansion matching that of the glass, and a flexiblesecond portion which is integrally attached to said first portion butwholly external of the glas casing.

The invention is hereafter described with specific reference to a glasscapacitor and in conjunction with the accompanying drawing in which:

FIG. 1 is an exploded sectional view of a glass capaci tor assembly,

FIG. 2 is an enlarged, partly in section, view of an electrical lead ofFIG. 1,

FIG. 3 is an elevational view in section showing the assembly of FIG. 1in consolidated form,

FIG. 4 is a fragmentary elevational view in section of the capacitorassembly of FIG. 3 and illustrating the formation of a fusion seal inaccordance with the present invention, and

FIG. 5 is a side view, partly in section, of a capacitor encapsulated inaccordance with the invention.

The capacitor assembly of FIG. 1 includes a capacitor element 12,opposed glass covers 1414 and composite electrical leads 16-16. Exceptfor leads 16, the components of the capacitor may be formed andassembled in accordance with procedures set forth in the earliermentioned Smith patent and prior patents there referred to. Details, notrepeated here, are incorporated from such patents by reference.

Capacitor element 12 is composed of thin layers of glass 18 interleavedwith metal foils 20. The latter are alternatively ofiset to provideterminal portions 22 extending beyond the glass. Additional layers ofmetal foil and glass may be provided if desired for additionalcapacitance. Glass covers 14 are arranged in opposed relationship toenclose capacitor element 12, with shaped lateral edges overlying theleads as taught by the Smith patent. The assembly is shown in explodedform, and dimensions are somewhat exaggerated and distorted, in order tobetter portray the arranged components.

Composite lead members 16 constitute a key feature of this inventionand, as shown in exaggerated form in FIG. 2, are composed of twodistinct, but integral, portions, flag member 24 and wire member 26.Flag 24 is a small rectangular piece of metal on the order of 20-35 milsthick and may, for example, have inch by inch sides, the actual sizebeing suitably proportioned to other capacitor components. It iscomposed of a base material 28, which may be a suitable ferrous alloysuch as 430 Ti stainless steel or an iron-nickel alloy, and surfacelayers 30 having a thickness of 1-2 mils, the surface layer beingcomposed to form a fused seal.

of a high conductivity metal such as copper applied by cladding orplating methods.

The flags are conveniently cut from a large strip or sheet of copperclad metal, thus providing unclad side sec tions. A base metal or alloyis selected to provide thermal expansion characteristics compatible withglass materials to which it is subsequently sealed. The copper clad hasno apparent effect in this respect presumably because there are uncladsurfaces on the flag member. A 52% nickel-48% iron alloy is preferredfor use with glasses having an expansion of 100-105 X 10' cm./cm./ C.since it provides an expansion match and is easily clad with copper.

The clad or plated surface layer is required to provide a lowresistivity path for electrical current flow between electrodes 22 andwire member 26. Without the clad layer, capacitor Q values are renderedprohibitively low by the higher resistivity of the base alloy requiredfor sealing purposes. The wire portion 26 of lead 16 may be of anyconventional flexible material, such as brass, copper or silver, that iscapable of being welded or otherwise integrally united to flag 24. Wiremember 26 of lead 16 may be butt welded to flag portion 24 on an uncladsurface. In assembling a capacitor, lead 16 is arranged with the copperclad surfaces 30 of flag portion 24 facing the overlying edge portionsof glass covers 14 and with one such clad surface electricallyconnected, as by welding, to electrode terminal 22.

The components shown in FIG. 1 are assembled in accordance with knownglass capacitor practice and consolidated into an integral unit bypassing the assembly through pressure rolls while heated to atemperature corresponding to the softening point of the glass which maybe on the order of 500 C. At this temperature, the glass componentsoften sufiiciently to adhere to each other and to the metal foils underthe influence of pressure. However, no appreciable flow of glass occursduring this operation as may be seen from FIG. 3 where glass covers 14are shown adhered to each other, but with their rounded edges 32substantially unchanged. At higher temperatures, where adequate glassflow for complete ealing would occur, the component layers formingcapacitor element 12 would be distorted and damaged. Accordingly, thecapacitor will not be a hermetically sealed unit at this stage.

The fused hermetic seal which characterizes the present invention isprovided in a separate sealing operation illustratively shown in FIG. 4.As shown in FIG. 3, there is in the consolidated and encapsulatedcapacitor a groove intermediate the rounded edges of glass covers 14. Inhermetically sealing the capacitor, this groove is filled with a softsealing glass 36 which has a sufliciently low viscosity at temperatureson the order of 450 C. or less Sealing glass 36 may be a well known softsealing glass, such as a lead borate glass, whose expansion will matchwith the expansion of the capacitor glass and flag 24. The sealing glassmay be flowed in molten form into the groove. More conveniently,however, it is powdered and applied in the groove in a suspension whichis then dried and fused as schematically shown in FIG. 4.

The fragmentary showing of FIG. 4 corresponds to the upper portion ofthe capacitor of FIG. 3 with sealing glass 36 fused to fill in thegroove and provide a smooth composite glass surface. As schematicallyshown, heat is applied locally, as by heating units 38, sealing glass 36to fuse such sealing glass and cause it to flow and fill the groove. Theamount of sealing glass employed and the arrangement of lead 16 shouldbe such that the sealing glass does not contact wire portion 26 of thecomposite lead member. Flag portion 24 will be imbedded in sealing glass36, but may extend slightly above the sealing glass surface. It is thefunction of sealing glas 36 to completely fill the groove or valleybetween glass covers 14 and therby form a fused hermetic seal betweenflag 4 portion 24 of lead member 16 and the glass cover members.

In this manner the desired hermetic encapsulation of a capacitor isprovided with a fused seal between the electrically conducting leadmembers and the glass encapsulation. The inflexible portion of the leadmember provides 1 the necessary expansion match for sealing to the glassencapsulation, while being embedded within the glass 1 Where flexibilityis immaterial. The flexible portion is out of contact with the sealingglass but provide the desired flexibility so that stresses are nottransmitted into the glass encapsulation of the capacitor. Furthermorethe Q value of the capacitor is maintained at the desired high .valuesattained with prior unsealed constructions.

While the invention has been described with reference l to a specificglass capacitor element, it will be understood that variousmodifications are possible within the scope of the invention. Inparticular, ceramic materials other 1 than glass may be employed asdielectric in the capacitor element, and the element may alternativelybe assembled by known metallizing procedures.

What is claimed is:

1. In a method for producing a glass encapsulated capacitor composed ofa capacitor element made up of electrodes separated by dielectricmaterial and electrically con: ducting leads attached to saidelectrodes. and extending outwardly through a glass casing thatencapsulates the element, the improvement which comprises attaching to.each electrode a composite electrically conducting lead composed of arelatively inflexible metal clad first portion, one end of which isattached to said electrode,.and a highly flexible second portionintegral therewith and extending from the other end of said firstportion, mounting the capacitor element between opposed glass covershaving lateral edges which overlie the clad portion of said lead,applying suflicient heat and pressure to produce an adherent, butsubstantially undistorted, unit, depositing a soft sealing glass in thesealing zone between the glass covers and around the clad metal portionof said lead, and applya ing localized heat to said sealing glass toproduce a fused glass seal between said lead and the glass casing, thehighly flexible portion of said lead being wholly external of said glasscasing.

2. In a method for producing a glass encapsulated .ca-. pacitor composedof a capacitorelement made up of elec-. t trodes separated by dielectricmaterial and electrically conducting leads attached to said electrodesand extend- 111g outwardly through a glass casing thatencapsulates theelement, the improvement which comprises attaching to each electrode acomposite electrically conducting lead 1 composed of a relativelyinflexible, metal clad first port tron, one end of which is attached tosaid electrode, and a highly flexible second portion integral therewithand ex tending from the other end'of said first portion, mounting thecapacitor element between opposed glass covers having lateral edgeswhich overlie the clad portion of said lead, applying sufiicient heat andpressure to produce an adherent, but substantially undistorted, unit,and applying a localized heat to form a fusion seal between said leadand l the glass casing, the highly flexible portion of said lead I beingwholly external of said glass casing.

References Cited by the Examiner UNITED STATES PATENTS 2,696,577 12/1954Smith 317261 2,699,594 1/1955 Bowne. 2,869,056 l/l959 Roovers 317-236 1OTHER REFERENCES Ser. No. 414,756, Long (A.P.C.) published April 1943.

JOHN F. CAMPBELL, Primary Examiner.

WILLIAM'I. BROOKS, Examiner.

2. IN A METHOD FOR PRODUCING A GLASS ENCAPSULATED CAPACITOR COMPOSED OFA CAPACITOR ELEMENT MADE UP OF ELECTRODE SEPARATED BY DIELECTRICMATERIAL AND ELECTRICALLY CONDUCTING LEADS ATTACHED TO SAID ELECTRODESAND EXTENDING OUTWARDLY THROUGH A GLASS CASING THAT ENCAPSULATES THEELEMENT, THE IMPROVEMENT WHICH COMPRISES ATTACHING TO EACH ELECTRODE ACOMPOSITE ELECTRICALLY CONDUCTING LEAD COMPOSED OF A RELATIVELYINFLEXIBLE, METAL CLAD FIRST PORTION, ONE END OF WHICH IS ATTACHED TOSAID ELECTRODE, AND A HIGHLY FLEXIBLE SECOND PORTION INTEGRAL THEREWITHAND EXTENDING FROM THE OTHER END OF SAID FIRST PORTION, MOUNTING